Hey guys and gals,
I've tried to ignore this leak for too long. It is an American Standard Reliant series shower valve, pressure balanced, that I've had for 15 years. It doesn't leak at any time except when the toilet is flushed. I just put in a new valve cartridge and it still does the same thing. Could it be the pressure balance assembly is causing the problem??

Welcome FarmerMark:
It may be something to do with the pressure balance system. The pressure does drop momentarily when you flush but your valve should stop leaking in a few seconds.
You might try Googleing American Standard and contact them. They answer emails quickly and give accurate answers.
Glenn